1. Field of the Invention
The invention relates to a method of producing a sensitive single-layer system for optical detection of the concentration of an analyte, such as, for example, oxygen. The analyte is capable of being investigated in the liquid or gaseous aggregate condition, or in a dissolved form, by absorption, luminescence or luminescence quenching of an indicator sensitive to the analyte. The indicator is preferably permanently immobilized in the optical beam path.
The invention further relates to a single-layer sensitive system including single-layer sensitive systems produced according to this method.
2. Discussion of Background Information
Systems and methods for determining the O.sub.2 partial pressure or O.sub.2 concentration by dynamic luminescence quenching of ruthenium-.alpha.-diimine complexes using optical waveguides have been described in EP 0 190 829, EP 190 830, EP 0 313 655, EP 0 344 313, DE 33 46 810, DE 37 02 210 and DE 41 08 808.
In this case [Ru(biPY).sub.3 ].sup.2+, [Ru(phen).sub.3 ].sup.2+, or [Ru(4,7Ph.sub.2 phen).sub.3 ].sup.2+ are used as a counter-ion in combination with chloride or perchlorate as a ruthenium complex whose phosphorescence is quenchable by oxygen. These indicator pigments are immobilized in polysiloxane matrix materials permeable to oxygen and therein are applied to glass fibers. Polysiloxanes with differing substituents and additives as well as specific layer sequences or protective membranes are proposed. Further, an arrangement of the indicator matrix system on the glass fibers is named, which can be ground at a specific angle.
In EP 190 829 a plurality of plasticisers are proposed as additives to the matrix material, which in this case is disposed on a fiber ground at an angle of 20.degree. to 30.degree.. As described in EP 190 830, the indicator matrix system can partly represent also the optical periphery of the fiber. In EP 0 313 655=WO 88/00339, without fastening on a specific material, an optically transparent matrix material is proposed, upon which the indicator is adsorbed or in which it is immobilized.
In EP 0 344 313 there is proposed a layer sequence of transparent substrate, indicator and gas-permeable membrane. According to DE 33 46 810, in addition to a plurality of indicators, among which are also found ruthenium compounds, and matrix polymers, among which are also polysiloxanes, special forms of the matrix, such as lamellae, spheres and films, but also solid additives such as silica gel, are proposed. In addition, various methods of introducing the indicator into the matrix are described, such as inward diffusion, mechanical mixing or covalent binding; additional protective layers are also named. The document DE-PS 37 02 210 describes the use of a two-component silicon polymer.
Finally, DE 41 08 808 proposes an indicator-containing silicon membrane convexly spread on a transparent substrate.
There is further described in DE 31 48 830 a device for determining the oxygen concentration in gases, liquids and tissues, in which a single-size layer is present on a transparent carrier with a luminescent surface formed with an adhesive or glue layer, an evaluable signal being intended to be obtained by irradiation with light at a specific wave length. The single-sized layer in this case is formed from a luminescent pigment itself or an inert carrier adsorbing this pigment, such as silica gel, or another water-rejecting plastic.
In GB 1 190 583 a gas sensor is disclosed, in which a luminescent material is taken up into a permeable or porous carrier matrix material, the matrix material preferably being a natural or synthetic polymer or porous glass.
There may be seen from EP 0 417 535 an optical oxygen sensor, in which oxygen indicator molecules are incorporated in a polymer. As an example of such a polymer, polydimethylsiloxane is disclosed, which is intended to be used as a carrier for the oxygen indicators. The polymers used are then dissolved in a solvent which contains the oxygen indicators. The solution is then applied in a thin layer to the substrate and hardened; only an indirect influence can be exerted on the layer thickness formed, and thus reproducible results are not always achievable.
In the sensor member described in EP 0 244 394 for determining material concentrations in gaseous and liquid samples, a carrier layer and an indicator layer with at least one indicator substance are named. In this case at least one photosensitive member is applied to the carrier layer.
A fiber-optic system for determining parameters in fluids is described in WO 94/10553. Here a fluorescent pigment is adsorbed from a solution onto a solid carrier, and, mixed with a liquid silicone, applied to a carrier; in this case for example polydimethylphenylsiloxane or unhardened polydimethylsiloxane may be used as a silicone in addition to others. The indicator matrix further contains indicator molecules on a carrier. A permeable membrane protects the indicator matrix, and the carrier in addition contains porous glass particles or a porous material based on silica gel or other porous glass particles and a carrier polymer of a non-ionic gel. Ru(1,10-phenanthroline) chloride is proposed as a possible indicator.
In the sensor member described in AT 390 678 for determining material concentrations, the use of an indicator layer consisting of polymer with a fluorescence indicator is proposed. The indicator layer is intended to consist of a porous glass layer, in which the indicator substance is immobilized. The indicator layer in this case can be a porous glass layer or a spun-on or rolled-on silicone layer, in which the indicator substance is present.
A similar device is also described in EP 0 578 630, in which case the sensor membrane of an optical sensor for determining a physical or chemical parameter of a sample is likewise intended to have a polymer matrix with an immobilized indicator substance.
In the optical sensor described in EP 0 354 204 , carrier particles of silica gel spheres are proposed, which are irreversibly bound to a fluorescence indicator.
The possible use of an adsorbent such as silica gel upon which a heterocyclic organic fluorescent dye stuff is adsorbed, can be seen in DE 28 23 318.
In Lippitsch, Max E. Et al.: "Fiber-Optic Oxygen Sensor with the Fluorescence Decay Time as the Information Carrier," in: Analytica Chimica Acta, 205, 1988, pp. 1 to 6, M. Lippitsch describes a fiber-optic oxygen sensor.
Exploitation of the alteration in luminescence for an oxygen sensor is likewise to be seen in an article by Wenying; Xu, et. al., "Oxygen Sensors Based on Luminescence Quenching: Interactions of Metal Complexes with the Polymer Supports," in: "Anal. Chem. 1994", 66, pp. 4133-4141. Here it is proposed to use the oxygen influence on [Ru(Ph.sub.2 phen).sub.3 ]Cl.sub.2 (Ph.sub.2 phen=4,7-diphenyl-1,10-phenanthroline) in combination with a polymer, such as, for example, polydimethylsiloxane. In addition silica gel is to be added, in order to be able to influence the sensor properties.
Ruthenium compounds are generally advantageous and appropriate for the named case of application due to their large Stokes Shift (large distance between energization and emission) and the relatively long-wave energization and emission; in this connection these are already widely described in the literature (see among others J. R. Bacon, K. N. Demas, "Determination of Oxygen Concentrations by Luminescence Quenching of a Polymer-Immobilised Transition-Metal Complex," Anal. Chem. 59, 2780-85, 1987).
Polysiloxanes are the polymers which have the greatest permeability for oxygen (see among others S. Egli, A. Ruf, A. Buck; Gastrennung mittels Membranen; Swiss. Chem. 6, 39-126, 1984).
Initially, a sufficient strength of the indicator-matrix systems is of great importance for practical use of such sensors. This is not sufficiently provided in the bonding of the indicator merely to the surface of a carrier. Furthermore, short response times, reproducible in the case of various examples of applicators, are to be achievable. The response times increase significantly, even in the case of polymers with good oxygen permeability, as the layer thickness increases or additional protective layers are applied.
It is therefore the object of the present invention to provide a method of manufacturing a sensitive single-layer system, and a sensitive single-layer system for measuring the concentration or the partial pressure of analytes, by means of which a reproducible and extremely short response behavior becomes obtainable.
According to the invention, the sensitive single-layer system is produced in such a way that the fluorescence indicators are adsorbed on to a filling material, and in connection therewith a mixture is produced with a matrix material permeable to the analyte to be investigated. The mixture produced is then compressed under the action of pressure, advantageously at an applied pressure of 12 to 20.times.10.sup.4 Pa, preferably 15.times.10.sup.4 Pa on a substrate, the layer thickness being formed in dependence on the applied pressure used. The sensitive layer thus applied is polymerized, polycondensed or hardened, this preferably being carried out in an extrusion mould to be used. The layer is additionally homogenized by swelling in a fluorescence indicator solution.
In order to swell the permeable matrix material, which is preferably polydimethylsiloxane, a methylene chloride solution containing a fluorescence indicator with a concentration of 10.sup.-1 to 10.sup.-6, preferably 10.sup.-3 to 10-4 molar concentration is used. The sensitive single layer system thus produced has in the sensitive layer an overall concentration of the fluorescence indicators in the permeable matrix of 10.sup.-1 to 10.sup.-6 mol/l, preferably 10.sup.-2 to 10.sup.-3 mol/l. Furthermore, a filler is also contained in the matrix with a weight proportion of 5 to 65% by weight, preferably 20 to 30% by weight. In this respect in an advantageous way silica gel porous glass should be used as a filler, upon which the fluorescence indicators are adsorbed.
Ruthenium-diamine complexes are advantageously used as fluorescence indicators, these more preferably being Ru(4,7ph.sub.2 phen).sub.3 Cl.sub.2 .times.5H.sub.2 O and being particularly suitable for detecting oxygen concentration.
An oxygen-permeable matrix material is for example polysiloxane and preferably polydimethylsiloxane.
The substrate material can be an optically transparent or reflective material and can be used in various forms. The sensitive layer can be applied to the substrate in accordance with the measuring purpose envisaged.
The sensitive single-layer system manufactured and designed according to the invention can be used for process monitoring in the most varied areas of technology and medicine, in order to monitor the concentration of analyte, particularly of oxygen, in almost any fluids, materials, mixtures of materials, and if necessary to utilize the measurement signals for influencing specific processes. Thus, in particularly the response time in millisecond range of the system according to the invention, in comparison to solutions previously known, is a positive factor. This is achieved by the capacity to produce extremely thin layers under pressure action with good adhesion and resistance. However, the most varied analyses may be carried out and supported, one example being the investigation of soil samples.
The sensitive single-layer system according to the invention can operate without additional adhesion promoters and protective layers. This is advantageously achieved by true-to-form polymerization, polycondensation or hardening and subsequent homogenization of the indicator matrix system on the substrate. An optical fiber or an optically transparent or reflective substrate can in this case preferably serve as a substrate.
A fiber optic applicator which may be manufactured according to the invention for a sensor system for optical detection of the concentration or of the partial pressure of an analyte, by adsorption, luminescence or luminescence quenching of an indicator immobilized in a permeable matrix material, is advantageously characterized in that good chronological durability and resistance in the utility medium even in flowing liquids, and further a signal level sufficient for good resolution, low response times, very low copy scatter of the values measured with various applications, and not least simplicity of manufacture, have been achieved.
The invention will be explained in more detail in the following with reference to an embodiment given by way of example.